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Ford Explorer - Ranger TPS Test Procedure

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Old 12-18-2009, 02:54 PM   #1
2000StreetRod
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Ford Explorer - Ranger TPS Test Procedure

The PCM relies on the throttle position sensor (TPS) to determine closed throttle for idle and all other throttle positions for acceleration or deceleration. A defective TPS can result in unpredictable engine behavior. Testing the TPS is easy and costs nothing. You'll need a voltmeter with a resolution of 0.1 volts or less. Some inexpensive voltmeters only have resolution to 0.5 volts. Also, an analog voltmeter measures continuously with an instantaneous display and is more likely to "catch" erratic characteristics. With the electrical connector connected to the TPS, backprobe (I use long straight pins) the wiper contact (gray/white wire) and the return (gray/red wire). Connect the positive lead of the voltmeter to the wiper probe and the negative lead to the return as shown in the photo below. Do not connect the voltmeter negative lead to ground instead of the return because the sensor return is connected to the PCM and not ground.
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Switch the ignition On but do not start the engine. The voltage should be approximately 1.0 volts with the throttle closed. Slowly rotate the throttle plate to wide open throttle (WOT) while watching the voltage reading. It should increase smoothly (with no dropouts or jumps) to at least 4.0 volts but less than or equal to 4.80 volts at WOT (mine read 4.64 volts).

If the TPS fails the above test move the voltmeter positive probe to the PCM provided reference voltage (brown/white wire) and measure the voltage. It should be approximately 5.0 volts. The PCM supplied reference voltage also goes to the fuel tank pressure sensor and the differential pressure feedback EGR (DPFE) sensor. If the reading is less than 1.0 volt one of the sensors may be shorted. Disconnect the TPS electrical connector and measure the resistance of the TPS.

Once the TPS tests good, I suggest that you perform the Idle adjustment procedure

Last edited by 2000StreetRod; 01-26-2010 at 07:08 AM.
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Old 06-16-2010, 06:38 PM   #2
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Dale,
Thanks for this! I just checked my TPS and I think it's O.K..
Idle= .777v and WOT= 4.69v. Close enough, right?




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Old 11-02-2010, 10:45 PM   #3
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Originally Posted by 2000StreetRod View Post
If the reading is less than 1.0 volt one of the sensors may be shorted. Disconnect the TPS electrical connector and measure the resistance of the TPS.

Once the TPS tests good, I suggest that you perform the Idle adjustment procedure
SO, what is the acceptable resistance range for the TPS?
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Old 11-02-2010, 11:08 PM   #4
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that 1.03volts is kinda of high at rest. just saying. if thats an odb 2 vehicle its really high. .8 or so is at idle.
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Old 12-08-2010, 11:48 PM   #5
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This saved me a lot of cash and got my truck back on the road.
THANKS
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Old 12-08-2010, 11:49 PM   #6
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This saved me a lot of cash and got my truck back on the road.
THANKS
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Old 03-27-2011, 01:23 PM   #7
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There's some things you should know about TPS settings. First, the normal range is .9 - 1.0 volts at idle, with about .98 being pretty typical. 4.64 volts at WOT is very common. Anything above 4.8 would probably trigger a bad TPS code. Same for anything less than .9.

There are 3 modes that the PCM categorizes the TPS position. Idle, part-throttle, and WOT. Idle is pretty much zero movement of the TPS. Part throttle is anything about .02 volts above idle and up, until you get to WOT. WOT is usually around 65 - 70% of the way to the floor. Despite the pedal not actually being totally floored, the PCM still sees these heavy throttle positions as WOT, because at that much throttle, the PCM is going open loop and commanding enrichment the same as it does at WOT anyway.

Setting the TPS idle is the most misunderstood part of the equation. First of all, TPS's are not adjustable. Some of them do have one of the two screw holes enlarged, which does actually provide a hair of "adjustment" room, but in all reality, Ford just enlarged that 2nd hole so that the screws were easy to put in, and the thing didn't have to be aligned perfectly. Some people, in certain circumstances, do enlarge the holes to make their TPS adjustable. This is only necessary when you need to jack up your minimum airflow screw (the throttle body stop screw) higher than stock, which puts the TPS too high, requiring TPS modification. It's doubtful you'll ever need to do this, unless you have a heavily modified engine.

The way the PCM determines what is idle and what is part throttle is a floating way, which is what's usually most misunderstood. When you turn the key to the ON position, the PCM reads the TPS voltage. Lets say yours is .98. The computer now stores that .98v as "idle", and anything .02 volts over that would engage part-throttle mode. So if your TPS idles at .93 volts, then .95v would be your part throttle. See how this varies? That works, right up until about 1.1v, where you're just flat getting out of the range of what the computer would ever see as idle. So if you have to modify your throttle body stop screw way up for some engine mod, you might have to modify the TPS to get it back down below 1.1v. But quite frankly, your tuner should be tuning the idle tables for larger cams and such.

The engine should just barely be able to breathe, with the IAC disconnected. When you unplug the IAC, it closes completely, so the engine would now breathe just thru the throttle body air gap. Most of the time, this minimum airflow screw should be set to where the car just barely stalls when you unplug the IAC, or at the most, so the engine just barely runs with it unplugged.

From there, the IAC adds some air (15 - 30%) to control idle up to normal idle speed. Then it adds another 15% or so to compensate for the A/C being turned on. Then it adds even more percentage of airflow to get the engine to idle fast when cold. As you can see, this extra airflow required by the IAC adds up to more and more duty cycle in a worst case scenario. The job of the minimum airflow screw is to add the minimum amount of air the engine itself will need to run, and the IAC picks it up from there and brings it to the desired idle speed.

In short, you should never have to mess with the minimum airflow screw. But if you have a modified motor, or feel it's been misadjusted already, then unplug the IAC and set the engine RPM so the engine either barely barely runs, or just stalls out. Then plug in the IAC and check the TPS voltage, to make sure it's in the .95 - 1.03 range, with .98 being your target.

A backwards method on a bone stock car might be to adjust the minimum airflow screw until your TPS says .98v. I have seen plenty of vehicles in that low 1.00 - 1.03 range, so if you're there, call it a day. Ford probably calls 1.0v it.

The TPS has several jobs. All have a big effect on the motor. The first job is to act like an electronic version of an accelerator pump. The PCM sees how fast your TPS changes, and how far it changes, and injects extra fuel in to compensate for quick throttle position changes. Basically an accelerator pump, but much more accurate.

The 2nd job is to determine the engine modes mentioned above. If you go from a part throttle position to idle (cruising and letting off the gas), the PCM knows you are decelerating, and can enter DFCO mode (deceleration fuel cut off). That's where, after a delay of approximately 7 seconds, the computer shuts off the injectors entirely to save fuel during coastdown. It turns the injectors on again at roughly 1200 rpm to prevent stalling.

Also, the PCM usually has different timing and fuel strategies for idle, so the PCM knowing when you're at idle is important. And then there's WOT. The computer needs to know when you're roughly 2/3 throttle and above, so it can start enriching the fuel mixture from stoich to about 15% - 20% richer than stoich.

The Ford IAC is a PWM (pulse width modulated) signal. The PCM throws voltage at a certain duty cycle to keep the valve open. Once that voltage is no longer sent, the IAC fully closes. This is why if you unplug the IAC, it closes. FYI, GM and Dodge normally use a stepper motor, and unplugging them will not cause the idle to drop. You would have to get them to electronically close before unplugging them, using a scan tool or tuning software.

The Ford IACs are very reliable, in my opinion. Normally, all they need is to be pulled off once in a blue moon and cleaned with carb cleaner and a brush, using care to not let the carb cleaner drain into the electronic half of the device.

Vehicles that have aftermarket cams in them that are lumpy often require more airflow at idle, to compensate for the reduced torque at idle that the new cam has. The tuner can add this extra airflow in the tune, via the IAC. If the IAC doesn't have enough extra room to still raise the idle up high when the engine is really cold and the AC is on, to bring the engine up to say 1800 RPM, then maybe it's time to add extra airflow at the throttle body stop screw (minimum airflow screw). But that might require TPS modification if you turn that screw out too much. Generally, this doesn't need to ever happen. But I do like the idea of setting the minimum airflow screw to where the engine just barely stalls. That ensures you're IAC will be in range, and makes for less work for the tuner in the idle airflow tables.
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Old 03-27-2011, 08:31 PM   #8
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Ratchet routine

Thank you for posting a very informative explanation of the Ford Strategy ratchet and other routines associated with the throttle position sensor. According to the 2000 MY OBD System Operation Summary the fault limits for the TPS are < 0.20 volts and > 4.80 volts. Many members have needlessly spent time trying to adjust their TPS to exactly 1.0 volts or some other slightly different value at closed throttle. I think it was a carry over from some comments posted on a Mustang forum. When the ignition is ON the ratchet routine executes periodically checking for a new lower voltage from the TPS. If one is detected then that value becomes the new closed throttle value. I don't remember if the closed throttle value is retained in keep alive memory.

I have my throttle stop set for the minimum speed the engine will run fairly smoothly with the IACV electrical connector disconnected. That way if the IACV fails I know that it has happened by the drop in engine idle speed but my engine doesn't die so I can continue to drive until I get time to fix the IACV.
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Old 03-28-2011, 03:52 AM   #9
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I'm not sure if closed throttle TPS voltage is keep-alive either. But you're right, the system periodically checks against your closed throttle and updates the voltage number if it's different. The important thing to remember is that it sees part throttle at just a couple hundreths of a volt above that learned closed throttle voltage.

There are tables in the PCM that tell the computer how much air to add, in order to meet the desired idle RPM in gear and in park/neutral. Then there are desired RPM settings themselves. Then there are A/C adders. And the dashpot stuff on decel is another set of tables or functions.

The stock tune bases their IAC settings on X amount of airflow coming thru the closed throttle body blade itself. This amount is set the way I spoke of earlier. If you go above this blade angle much, the PCM will be adding those other tables to this bigger number, causing the car to idle high in a variety of situations. The PCM does learn idle speed, and keep it in keep-alive memory, so even if you did jack this up, the PCM will learn it back down anyway. But the trick with tuning is to basically get the PCM to not have to learn things in the first place, by getting them right yourself.

In short, even if you're wrong, you'll likely never know it. If you have decel issues, or a general feeling that the idle or light cruise speeds vs throttle position are not quite right (like if you feel you are on cruise control when you are not), it might be necessary to back this down a bit. The Ford IACs don't go bad often enough to warrant worrying about them prematurely. Especially with occasional cleaning.
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Old 06-12-2011, 04:46 PM   #10
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US 2000 5.0L V8 high idle problem

About a week ago, I suddenly started having the same type of high idle problems being described in this thread. This is a bone stock 5.0 L with a K&N air filter, 120,000 miles. I checked and cleaned the IAC, but the problem continued -- idle at about 1600 RPM. I replaced the IAC -- same problem, no change. I suspected a vacuum leak, so I made a new gasket with no air holes and put it under the IAC, effectively eliminating IAC air. The engine idled smoothly at 500 RPM in Park and Drive -- OK, no vacuum leak. Re-install the normal IAC gasket. I disconnected the battery and reset the ECM -- no change, still high idle.
All of this started after I had the Motorcraft battery replaced under warranty at the Ford house. Is this just a coincidence, or could the battery replacement have somehow caused this problem -- I don't think so, but who knows. I do know that the Ford Tech used a keep-alive supplemental battery so the ECM and Radio would not dump their settings.
Should I go thru the "idle Adjustment Procedure"? I don't understand why this problem occurred in the first place -- I had not been monkeying around with the engine at all. Any suggestions would be greatly appreciated.
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Old 06-12-2011, 07:30 PM   #11
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Not a fix for a problem

The purpose of the idle adjustment procedure is not to fix a problem. It is to be performed to prevent the engine from dying should the IAC valve fail. Do you have any slack in your throttle linkage? Maybe the throttle plate is not closing because the cable is too short or the throttle body is dirty. Disconnect the throttle cable and see if the throttle plate closes freely against the stop.
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Old 06-13-2011, 12:15 AM   #12
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If you can rule out vacuum and intake gasket leaks for sure, and know that the only excess air entering the engine is thru the IAC, then the next step would be to see what would cause the PCM to raise the idle speed.

Generally, commanded idle speed is a function of engine coolant temperature. Idle is high (the colder the higher) when the engine is cold. As it warms up, idle speed drops to normal, and if it overheats, the idle speed kicks up a tad bit to drive the water pump and fan faster. So a bad coolant temp sensor could cause this. That should be rather easily discovered thru pulling codes, because in order to idle that high, the ECT sensor would have to be reading very cold. And if an engine doesn't warm up within a few minutes of starting, the PCM will throw a code saying 'engine coolant insufficient temp' or something like that.

Other mild idle speed adders are from a power steering pressure switch and an engaged a/c compressor. With a scan tool, you could look at commanded idle speed, IAC duty cycle, and engine coolant temp to see if they are reading reasonable results.

I would surely clean the throttle body first. I like to stuff a long strand of paper towels down the intake past the throttle body a little bit, and some more paper towels under the throttle body mouth to catch it dripping out. Then spray away, brush it's teeth, and gently pull the now nasty paper towels back out.

Another possibility is the TPS. If TPS voltage is much over 1.05v at idle, the PCM can see it as a part throttle condition and it could act goofy.




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Old 06-13-2011, 10:34 AM   #13
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The throttle moves freely and returns firmly to the stop with no binding or sticking. I will clean the throttle plate and body, although both appear to be very clean now. There is no soot, gum, or varnish on either part, and all feels smooth to the touch. Prior to my investigation of the IAC, there was no Check Engline Light; however, I caused it to come on when I started the engine with the IAC harness disconnected. I do not have a scan tool (yet!), but asked the guy at the parts house where I bought the new IAC to check the codes before he cleared them. The only code stored was P1504, "IAC out of range", which was to be expected. I will check the TPS voltages, Inlet Air Temp Sensor, and the Cooant Temp Sensor this morning.
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Old 06-13-2011, 03:42 PM   #14
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These days, an OBD2 scan tool is just about mandatory. There's just so much you can't do without one. Plus, they've gotten cheaper. Most low cost scan tools use generic obd2 data. Better tools will have "enhanced" data, meaning they bought a license from the manufacturer to use their proprietary data. This data is much higher quality, and covers more stuff. Generic obd2 only covers emissions related stuff. Enhanced covers things that don't necessarily affect emissions.

Better yet, get a scan tool that has all 9 modes. Most $100 - 200 tools only have the basic engine mode. Some have transmission too. Better tools will have all 9 modes, including ABS, airbag, body control modules, etc. While it's not the fastest refresh rate, I bought an OTC Nemisys because it came with all 9 modes, plus Ford, GM, Dodge enhanced OBD2 data. I later added Japanese enhanced data package. I paid like $500 for it, which was far cheaper than anywhere else.

For a more basic tool, I would look at an engine/trans scan tool made by Electronic Specialties maybe. Their top of the line is like $200. You could also look at obd interfaces for laptops, which have great datalogging and nice software, but also cost more to upgrade to enhanced data. Plus, you have to have the laptop in the car. That can be a pro or a con. The handheld is much faster and easier to use, but slower refresh rate. When I want seriously powerful data, I hook up my laptop and SCT Xcal3, which does all enhanced engine/trans PIDs and DMRs (data).

You'll pretty much need a scan tool with live datastream to read the coolant and IAT sensors, but you can read the TPS with a voltmeter. To do that correctly, you need some real thin backprobe pins, to poke into the back side of the harness connector. Shoving in regular voltmeter tips is a good way to damage the connector. A low buck way of doing this would be to use straightened paperclips as back probe pins, and then use the alligator clamp tips for your voltmeter probes.

TPS should be around .98v at idle. If it's under .9 or over 1.05, that's very suspect. Be sure to use both the power and ground from the TPS connector. If you ground the voltmeter to the engine or body, you don't have an actual PCM ground loop, and the accuracy could be off depending on ground offsets from the PCM to the battery. Just use the black TPS wire as a ground, and that issue goes away.




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Old 06-13-2011, 04:30 PM   #15
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Thanks for the info on scan tools. May be a while before I scratch up the bucks to buy one.
I checked the TPS voltage and it read 1.03 volts at idle and 4.67 V at WOT. There was barely enough slop in the TPS screw holes to adjust it down to 0.98 volts. After adjusting the throttle stop screw, the TPS voltage went down to 0.93 volts. Any further adjustment of the screw had no effect on TPS voltage. In fact I checked the clearance with a feeler gage and the stop was not even touching the screw. With the throttle blade sitting in the bore and not touching the stop screw, and the IAC air bleed blocked off, the lowest idle I coud achieve is 1000 RPM. I adjusted the stop screw until it barely touched the stop, replaced the IAC, and started the engine. The IAC opened to idle at about 1200 RPM, and then reduced idle to 1000 RPM, so it's functioning properly. I am now begining to suspect a vacuum leak somewhere, contrary to what I thought before.

A likely suspect is the PCV valve which has never been serviced or replaced since the Explorer was purchased in 2000, about 120,000 miles ago.
Uh, it's a LITTLE DIFFICULT to get to on the 5.0L V8! Is this one of those Service Manual sequences: "Step 1 -- Remove engine" type deals? Any tricks on how to get the PCV valve out? I think I can barely feel it with the tips of my fingers from the driver's side. Is it pushed into a grommet or screwed into the manifold?
I really love this Explorer with the V8, but my old '92 with the pushrod V6 certainly was easier to work on!!!
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Old 06-13-2011, 05:53 PM   #16
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You shouldn't really hae messed with that minimum airflow screw, but now that you did... a couple things you need to know. First, the throttle body and plate are teflon coated from the factory, to keep them from gumming up so easily. You're not supposed to clean it with any solvents. But, I have found that since I don't mind cleaning it once in a while, I usually don't pay attention to the sticker that says "dont clean me".

But once you clean it, you may have to clean it more often in the future. Next, that minimum screw shouldn't have been altered. It is more complicated than that, but basically here's the deal. The screw sets the minimum amount of idle RPM ever to be achieved. Usually, that's like 5 or 600 rpm. The IAC's job is to control the actual RPM, but minimum idle cannot be say 800, because that leaves no room for the IAC to go below that. In order for the IAC to work correctly, it needs room in both directions to move. Usually about 30% down and 70% upward. The PCM controls idle from 2000 rpm or so (high idle when really cold out) down to a less than stock idle, in case the RPMs are too high for some reason (vacuum leaks or something). So in order to give it room to work, we want about a 20 - 40% duty cycle at idle. You can see that in a scan tool.

Without that, just try adjusting the minimum airflow screw until you get a .98v TPS reading. Typically, most cars are about that unless they have a sticking throttle plate. When the throttle body is dirty and won't quite close all the way, you might see a 1.1v idle, which is too much due to the dirt. So before you do anything, clean the throttle body somehow (even if its just water and a paper towel, if its already nice and clean). Then, clean the plate edges themselves. Then, set the TPS to produce .98 at idle. That should get you close. If you had 1.05v before, probably it was just because of some crud on the throttle body or plate.

Also know that the minimum screw keeps the plate from binding in the bore when you let off the gas fast. Otherwise, it could get stuck closed. So for that reason, there should always be some opening of the throttle plate, just to keep it from sticking.

The elongated holes in the TPS, contrary to popular belief, are not adjustment slots. One hole is simply elongated to make it easier to install the TPS without it sticking to the screws too much. Granted, this light slop can be used to adjust the TPS, but you shouldn't need to if you have a stock cam profile.

Using your voltmeter, measure the TPS voltage if you let off the gas (engine off, full throttle, key on) nice and easy. See where the voltage stops. Then floor it and let off fast, snapping the throttle plate shut. See where the voltage stops there. If there is no dirt involved, both should end up at the same voltage within .01 or so. So clean the TB, set the TPS to .98 (or just loosen the screws, let it settle wherever it wants to, and then retighten the screws... no adjusting). The TPS should be right at 1v or slightly less. If not, find out why.

I suspect you do have a vacuum leak, and I remember that the PCV valves had some sort of vacuum hose leak issue on those models. Search the forums here for more info, but definitely suspect the hoses right around the PCV valve itself. From there, use some brakleen on an idling engine to carefully find any other vacuum or intake leaks. Also check that the intake tubing clamps are tight and there are no ripped or torn or leaking intake hoses.




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Old 06-14-2011, 07:50 PM   #17
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I really appreciate all of your help and suggestions. After an exhaustive investigation of the IAC & TPS settings and functionality, it became obvious that these devices were not causing the fast idle problem. So, I began to look for a vacuum leak, but was unable to find one. I suspected the PCV valve might be the problem since it has never been cleaned or replaced in 11 years (duh!). Went to NAPA and bought a new valve, went to "Serious Explorations" to find out just where the #@%! thing is located and how to get to it. After the engine cooled off a bit, I was able to cram my hand back between the engine and firewall, leaving bits of skin and hide hanging on various sharp projections and voila! I found it by feel and pulled the sucker out! The 2000 5.0L does not have the multi-legged vacuum hose arrangement that earlier V8s have, so it was not so difficult to pull it out once I found it. While I had it in hand I did the "SeaFoam treatment" recommended in the PCV valve thread -- WOW, what a bunch of smoke!
Anyway, replacing the PCV valve APPEARS to have solved my fast idle problem; however, it will take a few more start cycles and some mileage before the ECM resets to the new situation before I will know for sure.
Once again, thank you for all your help.
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Old 06-16-2011, 01:09 PM   #18
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Unfortunately, replacing the IAC valve, the PCV valve, and thoroughly cleaning and lubricating (with graphite) the throttle valve did not fix my fast idle problem completely. The engine will now eventually return to about 850 RPM at idle, but not upon deceleration from normal road speed. Instead, it "hangs" at about 1000 to 1200 RPM while decelerating and braking. The brakes are having to fight against the high idle engine speed, and the tranny is getting enough torque input to stay in gear longer which causes late and rough downshifts, including some U-joint clanking. I can put the tranny in Neutral at the stop sign, and the engine is still at 1000 - 1200 RPM for 10 seconds or so before it drops down to 850 RPM. This condition must be caused by the ECM holding the IAC valve open too much and/or too long during deceleration and braking (?). I disconnected the IAC and the condition goes away and the idle drops to about 600 RPM, but the old CEL is shining brightly in my face. Do I have an ECM problem? Maybe it's time to head for the Ford house for a diagnostic test $$$$$.
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Old 06-16-2011, 01:54 PM   #19
Pontisteve
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Well, Im sure Ford can fix it. But if you think about what's happening here, it should point you in the right direction. The PCM uses the IAC for 2 reasons. 1) To control idle speed anywhere from high idle (cold) down to below normal idle (for absolute idle control) and 2) as a dashpot, to make sure the car decelerates down to a stop correctly without stalling and also without feeling like its engine braking too much.

The throttle plate is set to a minimum idle (like 400 to 500 rpm) using the throttle body stop screw. The cable could also hold the throttle plate open, if something is wrong with it, like a sticking or melted cable. Check this! Cable should have some slack in it when the throttle is closed. Above this low idle speed, the PCM uses the IAC to bring idle up from the minimum 400 or so up to normal idle speed, or high idle if cold. In order for the IAC to be operating in it's intended range (15 - 40% duty cycle at warmed up idle), the throttle stop screw MUST have the throttle body idling by itself at that low low speed.

Disconnecting the IAC should result in a very low idle speed, and possibly even a stall. Normally a stall, really. The other guy on this forum bumps this a tad bit, so if the IAC does go bad, maybe the vehicle wouldn't stall. I would unplug the IAC, and drop the idle speed as low as you possibly can. This should coincide with a correct TPS voltage if all is well. A nice low idle speed should line up with the correct 1.00 volt TPS voltage.

Once you've got that right, and you're sure that the throttle cable isn't binding and holding it open (work the throttle cable slowly and see if it sticks when closing slowly), then the only other causes of high idle would be that the PCM is actually commanding this, or there is a vacuum leak.

Vacuum leaks likely produce lean codes in the PCM, and should be able to be found with some inspection under the hood. Remember that the PCV system is also part of that, so any air leaking into the inside of the engine (like valve covers, oil filler caps, etc) are a part of the vacuum system. In other words, make sure the oil cap is not missing.

Unmetered air is basically a vacuum leak too, and will cause a lean condition, which could raise idle speed. Make sure everything is hooked up right, and that all air entering the engine has gone thru the airflow meter first. This pretty much means check any aftermarket type stuff you've put on the car. The stock stuff is going to be in the right location already.

There is generally only two reasons why the PCM would command a high idle speed, and that is 1) the temperature is cold, or possibly too hot as well and 2) for dashpot purposes. So check the coolant temp sensor with a scan tool, or at the very least look for a trouble code related to the temp sensor. Next, make sure the TPS is in "idle" mode. The PCM reads the TPS voltage and classifies it as one of 3 modes. Idle, part throttle, and WOT. Anything over about 2.5 to 3.5 volts could be WOT, depending on the calibration in the PCM. Idle is whatever the TPS is when you turn the key on. Part throttle is anything that's about .03 volts over the idle voltage. This is where you could get into trouble. If the idle TPS voltage is 1.00, and your throttle plate sticks a little (or cable sticks), it might not return to 1.00. Maybe it gets stuck at 1.06, which would make the PCM think you're at part throttle.

At this point, if you cant find vacuum leaks, you need a scan tool with live datastream to read TPS voltage, TPS mode, and Coolant Temperature.

For the $100 you're going to spend at Ford, you could purchase a scan tool with live data. The top of the line Innova might be a good choice, albeit slightly more money. But at least you get to keep the tool when you're done, and it works for all 96-up cars.

It could also be an intake gasket leak, but the problem would be worse when you first start the car up.

There may also be a slim possibility of the vehicle speed sensor causing a deceleration high idle condition that you described, but I doubt it. If your speedo is always right, then you can rule that out.




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Old 06-17-2011, 03:43 PM   #20
2K-XLT
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OK, I went thru your entire procedure in detail:
TPS voltage was reading 0.98 volts.
Idle was at 1000 RPM.
I unplugged the IAC and the idle fell to about 600 RPM.
At this point, I plugged the IAC harness into my "old" IAC and laid it up on the intake hose next to the installed IAC. This was to keep the ECM happy. No CEL. Good.
I drove the vehicle to Kerrville and back, with a couple of stops along the way. About 60 miles R/T. The idle was reading about 600 RPM and the truck drove just fine with the IAC disabled. I like the lower idle speed. But, it's 100 degrees here today! May not be so good when it's cold out.
Got back home to Fredericksburg and let it cool off a while.
I set up my voltmeter to read TPS voltage again. Reading was 0.98 volts.
I then backed off the throttle stop screw until the TPS voltage quit falling at about 0.92 volts, then advanced the screw until it contacted the throttle stop and TPS voltage increased to 0.94 volts. This was to make certain that the throttle was stopping on the screw and not in the bore. Then I readjusted the TPS to read 0.98 volts. The engine idled at 450 RPM.
Turned engine off, plugged the IAC back in again, and started the engine. Idle was about 1100 RPM, but decreased in steps to about 850 RPM, it's lowest. Drove the truck around the neighborhood and it seemed to drive much better, even though the idle was at 1000 RPM when I stopped, but decreased to 850 RPM after a few seconds. So, I can probably live with it.
I still think I may have a problem with the ECM holding idle too high too long.
But, I ordered an Innova 3130 Scan Tool from Amazon.com last night. Should be here sometime next week. The saga continues.............
Thank you again so much for your expertise and helpful advice. There is no way I would have gotten this resolved without your help.
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